The present invention relates to multi-wavelength optical thermometry. Specifically, a non-contact temperature measurement is performed where the front and back surfaces of a workpiece, such as a semiconductor wafer, is used in an interferometric arrangement in order to measure changes in optical path lengths. Quite specifically, two laser beams, each having a different wavelength of light, are used for providing optical beams useful for measuring the temperature of a semiconductor wafer.
Measurement and control of the temperature of a substrate or wafer during many semiconductor manufacturing processes can greatly enhance yield. Preferably, non-contact methods are employed to avoid contamination during measurement and the problems associated with thermal contact. Also, the electronics used with the measurement instrument can be located remote from the vicinity where the manufacturing process is being performed.
Non-contact optical thermometry permits absolute determination of arbitrarily varying temperatures, opening up important applications, such as temperature control, when the temperature can be expected to vary from a set point.
Substrate temperature is widely recognized as an important processing parameter in the fabrication of a wide variety of thin film materials and devices, particularly in the microelectronics industry. Optical thermometry utilizes laser interferometry to determine temperature changes from the thermal expansion and refractive index changes of a transparent substrate whose front and back faces are polished and approximately parallel. Such techniques have been used to measure temperature of optically absorbing semiconducting materials, such as silicon and gallium arsenide, using IR lasers at wavelengths of 1.15 .mu.m, 1.5 .mu.m or 3.39 .mu.m.
The concept of optical temperature probes is well known in the art as evidenced by the articles entitled "An Interference Thermometer and Dilatometer Combined" by M. Luckiesh et al, J. Franklin Inst. 194, 251 (1922) and "An Interferometric-Dilatometer with Photographic Recording" by F. C. Nix and D. MacNair, Rev. Sci. Instru., Feb. 12, 1941, pp. 66-70.
More recently, in articles entitled "Wavelength Modulated Interferometric Thermometry for Measurement of Non-Monotonic Temperature Change", IBM Technical Disclosure Bulletin, 34, Oct. 5, 1991, pp 350-353 and "Thickness Measurements Using IR Tunable Laser Source", IBM Technical Disclosure Bulletin, 35, 1B, June 1992, pp 465-468, there are disclosed a laser based arrangement for temperature measurement in which infrared laser radiation illuminates a silicon wafer and is reflected from both the front and back surfaces of the silicon workpiece. The workpiece is transparent because of the semiconductor band gap, has a dielectric constant of approximately 12 and is quite temperature dependent. As the workpiece temperature changes, the path length through the workpiece changes primarily from a shift in the dielectric constant. The resulting interference signal formed in the cavity between the two silicon workpiece surfaces also changes. In the arrangements described in the above articles, the laser frequency dithers slightly to provide a derivative signal indicating whether the temperature is increasing or decreasing. The temperature is then calculated by fringe counting at a rate, for silicon, of 7.degree. C. per fringe.
The complexity of counting fringes over a process temperature range in the order of hundreds of degrees will be evident to those skilled in the art. Moreover, errors result if the rate of change of the temperature is greater than the rate at which the measurement scheme can count the fringes. If the temperature could be measured using a single fringe over the entire temperature range, the ambiguity resulting from fringe counting could be eliminated.